1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating a metal gate of the semiconductor device, and more particularly, to a complementary metal oxide semiconductor (CMOS) device having metal gates with different work functions and a method of fabricating the metal gate of the CMOS device.
2. Description of the Related Art
A complementary metal oxide semiconductor (CMOS) device includes a p-channel metal oxide semiconductor (PMOS) transistor and an n-channel metal oxide semiconductor (NMOS) transistor that are complementarily operated. Thus, the CMOS device can increase overall efficiency and operation speed, and accordingly can be used as a high performance semiconductor device since the CMOS device has the same characteristics as a bipolar transistor.
In the prior art, a gate of the CMOS device is fabricated, for example, by forming a polysilicon, which is doped in same type as the channels, on a gate insulating film formed of a silicon oxide (SiO2) film. For example, a gate of the PMOS transistor is formed using a polysilicon doped with a P-type dopant, and a gate of the NMOS transistor is formed using a polysilicon doped with an N-type dopant. However, as the thickness of thin films in a semiconductor device is gradually reduced due to increased integration of the semiconductor device, the thickness of the conventional SiO2 thin film that is used as a gate insulating film is reduced. As a result, a leakage current is increased due to tunneling effect, and thus, power consumption is increased above a standard value. In order to address this problem, high-k oxide films have drawn attentions since the high-k oxide films can be used to realize a thick gate insulating film that electrically has an identical equivalent oxide film thickness and physically does not generate tunneling. As a result, SiO2 is replaced by high dielectric materials such as HfO2 or Al2O3.
However, when the high dielectric material is used for the gate insulating film, dopants in the doped polysilicon formed on the gate insulating layer are diffused into the gate insulating film. In order to address this problem, recently, studies to use a metal gate instead of the doped polysilicon have been conducted. When a metal is used for the gate, in order to obtain a desired threshold voltage of the NMOS transistor and the PMOS transistor, it is necessary to use two metals having work functions different from each other with respect to the NMOS transistor and the PMOS transistor as the gate electrode. However, a method of fabricating this structure is complicated and difficult.